Pulse repetition frequency determination system

ABSTRACT

1. A system for determining the pulse repetition frequency of a relatively short train of pulses comprising: an input circuit adapted to receive said train of pulses: selectively adjustable delay means coupled to said input circuit for delaying said pulses a predetermined amount; selectively adjustable gate pulse generating means coupled to said delay means for generating gate pulses having a predetermined length responsive to the delayed pulses; and coincidence means coupled to said gate pulse generating means and to said input circuit for providing an output signal responsive to coincidence of an input pulse and a gate pulse whereby the delay of said delay means indicates the repetition frequency of said train of pulses.

lJnitedStates Patent 1 1 1111 3,758,856

Fromm 1 Sept. 1 l, 1973 54 PULSE REPETITION FREQUENCY 2,716,189 8/1955 Ayres 328/138 DETERMINATION SYSTEM 2,824,958 2/1958 Dunn 328/109 2,902,649 9/1959 Bachman 324/88 x inventor: Kenneth Frnnim, Cincinnati, 2,905,887 9/1959 Taddeo 324/68 01110 2,939,075 5/1900 Schwab 324/68 x 2,951,985 9/1960 Hudson et a1 324/88 [73] Assignee: International Telephone and Telegraph Corporauon Nutley' Primary Examiner-Benjamin A. Borchelt [22] Filed: Apr. 3, 1959 Assistant Examiner-G. E. Montone 1 pp NO 804 045 Att0meyPercy P. Lantzy and Gust and lrish EXEMPLARY CLAIM [52] 324/7871" l. A system for determining the pulse repetition fre- 51 l t Cl 23 02 quency of a relatively short train of pulses comprising: z i 5 an input circuit adapted to receive said train of pulses:

selectively adjustable delay means coupled to said input circuit for delaying said pulses a predetermined amount; selectively adjustable gate pulse generating means coupled to said delay means for generating gate pulses having a predetermined length responsive to the 328/109,1ll, 129,138, 1l2,l40;343/7.5, 17.7,18, 8, 9,18 E, 18 R, 7.7; 324/78, 68, 88, 78 R, 185; 329/107 [56] References Cited delayed pulses; and coincidence means coupled to said UNITED STATES PATENTS gate pulse generating means and to said input circuit 3,714,654 1/1973 Wicks 343/18 E for providing an output signal responsive to coinci- 2,545,464 3/1951 Hoeppner et a1. 2,546,972 4/1951 Chattersea et al. 2,560,289 7/1951 Hansbrook 328/111 dence of an input pulse and a gate pulse whereby the 250/27 delay of said delay means indicates the repetition fre- 324/68 x quency of said train of pulses.

2,648,766 8/1953 Eberhard 328/110 2,688,051 8/1954 Liquori et a1. 324/68 X 10 Claims, 6 Drawing Figures 4 g 7 6 8 9 /.2 a 1K E1 M nly 1/ 1 7 647! a: Mm

3 /7 U s N Pl/zs'i Gilt/609702 M AND 4 4 PATENIEUSEPI 1 ms SHEET 1 0F 3 GATE PULSE,

PULSE REPETITION FREQUENCY DETERMINATION SYSTEM This invention relates generally to transmission systems utilizing a relatively short train of pulses, for example, pulsed radar, and more particularly to a system for determining the pulse repetition frequency of such a train of pulses.

There are instances in which it is desirable to provice apparatus for detecting the presence of a pulsed radar signal and more particularly for determining the identity and nature of a particular radar signal. While there are many means of radar signal selection or discrimination, a particularly convenient method for identifying a particular radar signal is by determination of its pulse repetition frequency (PRF). While determination of the PRF of a particular pulsed signal can be readily accomplished by the use of tuned circuits since the PRF has a fundamental frequency, this method of PRF determination requires that the signal being measured be present for an appreciable period of time. Thus, in the case of scanning-type radars, it will generally be found that an insufficient number of pulses are provided during the brief interval that the radar beam is received during each scan to permit determination of the PRF by the use of resonant circuits; depending on the beam width, scanning rate and PRF of the particular radar, as few as pulses may be received during each interval that the radar beam illuminates the receiving antenna. It is therefore desirable to provide a system for determining the PRF of a train of relatively short pulses, such as a radar signal, such system being capable of making PRF determination responsive to a very small number of pulses, such as are received from a scanning-type radar. It is further desirable that such a system be capable of operation over a wide range of PRF, i.e., for example 200 through 2,000 pulses per second, reject harmonics and sub-harmonics of the desired signal, and possess the ability to select low PRFs in the presence of high PRFs. Since the system may well be air-bome, it is obviously desirable that it be simple, require a minimum of attention with adjustments being made without undue burden on the operator, and

that the circuitry not be unduly complex.

In its broader aspects my invention provides an input circuit adapted to receive the short pulses, the input circuit being coupled to delay means for delaying the pulses a predetermined amount. The delay means is further coupled to coincidence means which is also coupled to the input circuit. Thus, the first pulse of the input-signal triggers a delay and if the output pulse from the delay occurs in coincidence with a succeeding input pulse, an output signal is provided; the delay of the delay means therefore indicates the repetition frequency of the train of pulses. More specifically, 1 provide gate means coupled to the input circuit for providing a trigger pulse responsive to an input pulse, the gate means being turned off responsive to reception of an input pulse. The gate means is coupled to the delay means which in turn is coupled to gate pulse generating means whereby gate pulses having a predetermined length, at least as long as the input pulses are provided responsive to the delay pulses. The gate pulse generating means is coupled to the gate means whereby the same is turned on for reception of another input pulse responsive to the gate pulse, thegate pulse generating means being also coupled to the coincidence means so that an output signal is provided responsive to coincidence of an input pulse and a gate pulse.

It is therefore an object of my invention to provide improved means for determining the PRF of a train of relatively short pulses.

Another object of my invention is to provide an improved system for determining the PRF of a train of relatively short pulses which is operable on short-pulse bursts.

A further object of my invention is to provide an improved system for determining the PRF of a train of relatively short pulses which provides for rejection of harmonics and sub-harmonics of the desired PRF.

The above-mentioned and other features and objects of this invention and the manner of attaining them will become more apparent and the invention itself will be best understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a block diagram illustrating my improved PRF determination system;

FIG. 2 illustrates the relationship of the PRF period and the delay and is useful in explaining the mode of operation of my system;

FIG. 3 illustrates in chart form the time relationship of the various signals employed in my system;

FIG. 4 is a schematic circuit diagram of the preferred embodiment of my improved PRF determination systern;

FIG. 5 is a chart which illustrates the method employed in eliminating sub-harmonics with my system; and

FIG. 6 illustrates a system employing two of my PRF determination systems which is insensitive to subharmonics as well as to harmonics of the desired PRF.

Referring now to FIG. 1, my improved PRF determination system, generally identified as 1, includes an input circuit 2 comprising a suitable antenna 3 adapted to receive a pulsed radar signal 4 with conventional detector means 5 coupled thereto. A gate circuit 6, which may be a bistable multivibrator, is coupled to the input circuit 2; the first input pulse 7 fires the gate circuit 6 to provide a positive step output or trigger pulse 8. The gate circuit 6 is arranged so that it is incapable of being retriggered responsive to further input pulses until it has subsequently been retriggered by a delayed gate pulse, as will be hereinafter more fully described. The trigger pulse 8 is differentiated, and the differentiated trigger pulse 9 triggers delay means 10 to provide a delay pulse 11 having a predetermined time delay D with respect to the input pulse 7. In the preferred embodiment of my invention, the delay means 10 comprises a sweep voltage generator 12 and a voltage com parator 13; the differentiated trigger pulse 9 triggers the sweep voltage generator 12 to initiate the sweep 14 which is compared with a selectively adjustable predetermined voltage level in the voltage comparator 13, thereby to provide the delayed output pulse 11 when the sweep voltage 14 reaches the predetermined voltage level.

The comparator 13 is coupled to pulse generator 15 which provides a gate pulse 16 having a predetermined length responsive to the delayed pulse 11; the gate pulse 16 has the same polarity as the input pulse 7 and has a length at least as long as the input pulse. Pulse generator 15 is coupled to a coincidence or AND circuit 17 which is also coupled to the input circuit 2.

Thus, if the gate pulse 16 provided by the pulse generator 15 finds coincidence with a succeeding input pulse 7a, the AND circuit 17 provides an output signal which may be applied to the intensifier element of a suitable indicator tube 18. Pulse generator 15 is also coupled to the gate circuit 6 and supplies another gate pulse 19 thereto having a polarity. opposite from that of the input pulse 7, thus retriggering the gate 6 for reception of another input pulse 7.

Referring now to FIGS. 2 and 3 in addition to FIG. 1, it may not be necessary that the system of FIG. 1 precisely determine the PRF of the pulse signal 4 but merely determine whether it is within a given range. Thus, the delay means may be arranged to provide the delay D T AT where T is the predicted period of the input pulses 7, and the pulse generator may be arranged to provide gate pulses 16 and 19 having a length ZAT. It is thus seen that if the next gate pulse 7a following the one which triggered the gate 6 to provide the trigger pulse 8 finds coincidence in the AND circuit 17 with the gate pulse 16, an output signal will be provided for providing indication on the indicator tube 18. It will be seen that applying the inverse polarity gate pulse 19 to the gate 6 thereby to turn it on renders the system capable of accepting a train of pulses rather than a single pulse, since turning on of the gate 6 occurs just prior to the next expected pulse 7a; reception of the pulse 7a by the gate 6 again turns the gate off and the above described mode of operation is repeated. Reference particularly to FIG. 3 will show that an initial input pulse 7 initiates step output 8 of gate 6, the gate being turned off simultaneously, the step output 8 of the gate 6 in turn initiating sweep delay voltage 14. When the sweep voltage 14 provided by sweep voltage generator 12 reaches the selectively adjustable voltage E of the comparator 13, the gate pulse generator 15 is triggered to provide gate pulses l6 and 19, the gate pulse 19 being differentiated and applied to the gate 6 to turn on the gate as at 20. Since the next successiVe input pulse 7a together with the gate pulse 16 are both applied to the AND circuit 17, it will be readily seen that coincidence of the next input pulse 70 and the gate pulse 16 in the AND circuit 17 will result in the provision of an output pulse 21 to the indicator tube 18.

It will now be seen that sub-harmonics of the input pulses 7 (sub-harmonics are here referred to as those having one-half, one-third, one-fourth, etc. of the period T of the input pulses 7) will provide an indication on the indicator tube 18. Thus, in the case of a signal having a sub-harmonic T/2 of the expected period T, the next successive pulse 7b as shown in FIG. 3 will not find coincidence with the gate pulse 16 and thus will provide no indication. However, the second successive input pulse 7c will find coincidence with the gate pulse 16 and will provide an output indication 21. It will be seen however that a harmonic of the extended pulse period T (harmonics are here referred to as those having a period which is a multiple of the exected period T, for example 2T, 3T, 4T, etc.) will not provide an indication; a signal having a period 2Tso that the next successive input pulse is 7d will result in the pulse 7d not finding coincidence with the gate pulse 16 so that no output signal is provided.

Referring now to FIG. 4, the preferred embodiment of the system of FIG. 1 is shown; Detector 5 of the input circuit 2 is connected by coupling capacitor 22 to control grid 23 of one tube 24 of bistable multivibrator 6. The plate 25 of tube 24 is connected by a suitable plate resistor 26 to a suitable positive source of direct current potential, shown here as being battery 27, and the cathode 28 is connected to ground as shown and to cathode 2 of the other tube 30. Grid 23 of tube 24 and grid 31 of the other tube are connected to ground by bias resistors 32a and 32b and the plate 33 of the other tube 30 is likewise connected to battery 27 by plate resistor 34. Grid 23 of tube 24 is connected to plate 33 of tube 30 by parallel-connected capacitor 35 and resistor 36 while grid 31 of tube 30 is connected to plate 25 of tube 24 by parallel connected capacitor 37 and resistor 38.

It will now be seen that application of the positivegoing input pulse 7 to the grid 23 of tube 24 will drive grid 23 positive, thus causing tube 24 to conduct heavily. The plate current flow of tube 24 flowing through plate resistor 26 provides sufficient voltage drop to bias grid 31 of tube 30 sufficiently negative through resistor 38 and capacitor 37 so that tube 30 is cut off. Tube 30 being cut off causes its plate potential to increase to approximately that of the battery 27 and by virtue of the connection of the plate 33 to the grid 23 of tube 20 through resistor 36 and capacitor 35, grid 23 will remain positive so that tube 24 continues to conduct after the input pulse 7 has been removed from the grid 23.

Plate 33 of tube 30 of multivibrator 6 is also connected to delay sweep circuit 12 by differentiating circuit 39 comprising capacitor 40, diode 41 and resistor 42; capacitor 40 and diode 41 are serially connected between plate 33 of tube 30 of bistable multivibrator 6 and grid 43 of tube 44 of the sweep voltage generator 12. Resistor 42 is connected between the grid 43 of tube 44 and ground as shown. Thus,'the stepped voltage 8 provided by the multivibrator 6 when triggered by input pulse 7 is difierentiated to provide differentiated trigger pulse 9 applied to the grid 43 of sweep voltage generator tube 44.

The plate 45 of tube 44 is connected to the positive source of potential 27 by a suitable plate resistor 46 and cathode 47 is connected to ground by cathode resistor 48. Cathode 47 is also connected to point 49 between serially connected resistor 50 and capacitor 51 connected between ground and the positive source of potential 27 as shown. Plate 45 of tube 44 is connected to grid 52 of voltage comparator tube 53 by capacitor 54 with capacitor 55 also connecting plate 45 to ground and with clamping diode 56 and resistor 57 connecting grid 52 of comparator tube 53 to ground; capacitors 54 and 55 provide the sweep voltage 14 and clamping diode 56 which prevents a negative excursion of the voltage thereacro'ss provides sweep voltage 14a.

Plate 57 of voltagecomparator tube 53 is connected to the positive source of potential 27 and cathode 58 is connected to ground by serially connected resistors 59 and 60. The voltage atwhich comparator tube 53 will conduct and thus provide delayed pulse 11, is provided by adjustable resistor 61 connected between the positive source of potential 27 and point 62 between resistors 59 and 60, resistor 60' also being shunted by capacitor 63. It is thus seen that selective adjustment of the variable resistor 61 varies the potential applied between the cathode 42 and plate 57 thereby to determine the point on sweep voltage 14a at which comparator tube 53 will conduct.

Comparator circuit 13 is coupled to pulse generator 15 by a cathode follower connection from point 53 through coupling capacitor 64 to grid 65 of tube 66. Plate 67 of tube 66 is connected to a suitable source of positive plate potential, such as battery 68, by'a plate resistor 69 and cathode 70 is connected to ground through cathode resistor 71 and also to cathode 72 of tube 73. Grid 65 of tube 66 is also connected to ground through biasing resistor 74 as shown. Grid 75 of tube 73 is coupled to plate 67 of tube 66 by parallelconnected capacitor 76 and resistor 77 and plate 78 of tube 73 is likewise connected to the positive source of plate potential 68 by plate resistor 79.

A negative-going gate pulse 19 is provided from the pulse generator 15 by coupling plate 67 of tube 66 through differentiating circuit 80 to grid 23 of tube 24 of the multivibrator 6; differentiating circuit 80 comprises serially connected capacitor 81 and diode 82 and resistor 83 connecting the midpoint between capacitor 81 and diode 82 to ground as shown.

It will now be seen that the leading edge of the gate pulse 19 provided by the pulse generator 15 responsive to the delay pulse 11 is differentiated by the differentiating circuit 80 to provide a negativegoing differentiated pulse 19a which in turn is applied to the grid 23 of the multivibrator circuit 6. Negative-going pulse 19a drives grid 23 of tube 24 sufficiently negative to cause tube 24 to be cut off. Cutting off of tube 24 causes the potential of plate 25 to raise to practically that of battery 27 which causes a corresponding increase in the potential of grid 31 of tube 30 by virtue of its connection to plate 25 of tube 24 and through capacitor 37 and resistor 38. Increasing the potential of grid 31 of tube 30 causes tube 30 to conduct heavily, thus resetting the multivibrator circuit 6 for reception of the next gate pulse 7a.

Gate pulse generator 15 is coupled to AND circuit 17 by coupling capacitor 84 connected between plate 78 of tube 73 and grid 85 of tube 86. Plate 87 of tube 86 along with plate 88 of tube 89 is connected to a suitable source of positive plate potential, shown here as battery 90, and cathode 91 of tube 86 is connected to ground through cathode resistor 92. Grid 93 of the tube 89 is connected to grid 23 of tube 24 of the multivibrator 6 and to the input circuit 2 by connection 94 and is also connected to ground through resistor 95. Cathode 96 of tube 89 is also connected to ground through cathode resistor 97 and plate 88 of tube 89 is connected to ground through serially connected resistor 98 and potentiometer 99. Movable tap of potentiometer 99 is connected to grid 85 of tube 86 by resistor 101, adjustment of adjustable tap 100 of potentiometer 99 determining the duration or length of the gate pulse 16 applied to grid 85 of tube 86. Midpoint 102 between resistor 98 and potentiometer 99 is connected to cathode 91'of tube 86 by serially connected resistor 103 and diode 104 and midpoint 105 between resistor 103 and diode 104 is connected to cathode 96 of tube 89 by diode 106 and also to output terminal 107; output terminal 107 is adapted to be connected to any suitable indicating device, such as the intensifier element of indicator tube 18.

Multivibrator circuit 6, delay sweep voltage generator circuit 12, comparator circuit 13, pulse generator circuit 15, and the AND circuit 17 will be recognized as individually conventional and it will be readily understood that other suitable and conventional circuits may be employed in each instance. in the case of the employment of a sweep voltage generator 12 and comparator 13 for providing the selectively adjustable delay pulse, this combination was employed so that the recovery time could be made less than the minimum pulse width of the input signal 7, which would not be possible with a simple monostable multivibrator delay. it will, however, be readily understood that other delays such as a conventional bootstrap delay may be employed.

As indicated heretofore, the system of FIGS. 1 through 4, inclusive, is sensitive to sub-harmonics of the desired PRF. Thus, referring to FIG. 5, it will be seen, for example, that if the system is adjusted for a predetermined period T and the actual period of the input signal is also T, an output indication will be provided. However, it will further be seen that with a setting (s) of a period T, an output indication will be provided with input signals having periods T/2, T/3, T/4, T/5, T/6, T/7, etc. It will furthermore be seen that if the input signal has a period T, with the systems having a setting corresponding to T/2, no indication is provided since the period T of the input signal is actually a harmonic of the setting T/2. Thus, the actual period of an input signal having a period T, as distinguished from a sub-harmonic, may be determined by selectively changing the setting S of the system from T/7 through T, and it will be observed that at a setting T/2 no output will be provided, whereas at a setting T, an output will be provided. Thus, with the use of a single system as shown in FIGS. 1 through 4, progressive adjustment of the delay of the system from a sub-harmonic toward the expected period of the input signal provides an indication of the period of the signal when the output indication changes from no indication to a coincidence indication. It will be seen in FIG. 5 that the boxes marked indicate an output and those marked indicate an absence of an output from the system under a given set of conditions with the boxes marked in circles indicating a correct PRF indication while those not so marked being incorrect. Thus, if the system setting S is set for a PRF period T, it will produce an output for input signals I of T, T/2, T/3, T/4, etc. of which only Tis correct; all signals having periods longer than T, i.e., the harmonics 2T, 3T, etc. produce no output.

Reference to FIG. 5 above indicates a convenient means for eliminating sub-harmonic signals (T/2, T/3, etc.), and such a system is shown in FIG. 6. Here, a first PRF determining system 110 of the type shown in FIGS. 1 through 4 is provided set for the desired period T, and a second indentical PRF determining system 1 1 1 is provided set, however, for the sub-harmonic T/2. The input circuits of systems 110 and 111 are connected to a common input circuit 112 and an anticoincidence circuit 113 is provided having one of its input circuits coupled to the input circuit 112 and its other input circuit coupled to the output circuit of PRF determining system 111. The output circuit of the PRF determining circuit 110 and the output circuit of the anticoincidence circuit 113 are coupled respectively to the input circuits of another coincidence or AND circuit 1 14. It will be readily seen that the system of FIG. 6 will provide a indication when the input signal has period T, while input signals having either a sub-harmonic or a harmonic of the desired period T will provide either -H- or output indications. In the system of FIG. 6 the PRF determining system 110 being set for the expected period T will provide output signals, as shown in FIG. 5, in response to input signals having periods of T and sub-harmonics T/2, T/3, etc., however, by virtue of the provision of anti-coincidence circuit 113, an output signal is provided only if the input pulse occurs in the absence of an output signal from the PRF determining circuit 111. Reference again to FIG. will show that with the PRF determining circuit 1 1 1 set for an expected period T/2, if the input signal in fact 'has a period of T, there will be an absence of an output signal from the system 111 when the next successive input pulse arrives, and thus the anti-coincidence circuit 113 will pass the next successive input pulse which will find coincidence with the output signal from the PRP determining circuit 110 'in the AND circuit 114 to provide a resulting output signal indicating that the input signal in fact has a period T. On the other hand, if for example the input signal is the sub-harmonic T/2, the PRF determining circuit 111 will provide an output signal and therefore the anti-coincidence circuit 113 will not provide an output signal to the AND circuit 114 so that no output signal will be provided. It will be readily apparent that the two PRF determining systems 110 and 111 may be readily ganged, as at 1 since this would merely require ganging of the two variable resistors 61 of FIG. 4. Thus, the period T of the system 110 and T/2 of the system 115 can simultaneously be varied in order to determine the actual period of the input signal; it will be seen that it is merely necessary that the two delay circuits of the two systems 110 and 111 produce delays in the ratio of 2 to 1.-

It will now be seen that I have provided an improved PRF determination circuit characterized by its comparative simplicity, ease of adjustment, and in its simplest form, insensitivity to harmonics of the PRF. It will also be seen that two of my improved systems may be readily combined in order to provide a system which is insensitive not only to'harmonics of-PRF, but also to sub-harmonics. It will now be seen that my improved system provides for the rejection of both harmonics and sub-harmonics of the desired PRF, provides a broad range of PRF coverage and further, and most importantly is operable on short pulse trains of a scanning radar in contrast with prior filter circuits which require a burst of substantial duration inorder to effect a PRF determination.

While I have described above the principles of my invention in connection with specific apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of my invention.

What is claimed is:

l. A system for determining the pulse repetition frequency of a relatively short train of pulses comprising: an input circuit adapted to receive said train of pulses; selectively adjustable delay means coupled to said input circuit for delaying said pulses a predetermined amount; selectively adjustable gate pulse generating means coupled to said delay means for generating gate pulses having a predetermined length responsive to the delayed pulses; and coincidence means coupled to said gate pulse generating means and to said input circuit for providing an output signal responsive to coincidence of an input pulse and a gate pulse whereby the delay of said delay means indicates the repetition frequency of said train of pulses.

2. A system for determining the pulse repetition frequency of a relatively short train of pulses comprising: an input circuit adapted to receive said train of pulses; delay means coupled to said input circuit for delaying the input pulses by an amount D T -AT where D is the time delay, T is the predicted period of the input pulses and AT is a period at least one half the duration of one input pulse; gate pulse generating means 'coupled to said delay means for generating gate pulses having a length 2AT responsive to the delayed pulses, said gate pulses being at least as long as said pulses; and coincidence means coupled to said gate pulse generating means and to said input circuit for providing an output signal responsive to coincidence of an input pulse and a gate pulse whereby the delay of said delay means indicates the repetition frequency of said train of pulses.

3. A system for determining the pulse repetition frequency of a relatively short train of pulses comprising: an input circuit adapted to receive said train of pulses; gate means coupled to said input circuit for providing a trigger pulse responsive to an input pulse, said gate means being turned off responsive to reception of an input pulse; delay means coupled to said gate means for delaying the trigger pulses a predetermined amount; gate pulse generating means coupled to said delay means for generating gate pulses having a predetermined length responsive to the delayed pulses, said gate pulse generating means being coupled to said gate means whereby the same is turned on for reception of another input pulse responsive to a said gate pulse; and coincidence means coupled to said gate pulse generating means and said input circuit for providing an output signal responsive to coincidence of an input pulse and a gate pulse whereby the delay of said delay means indicates the repetition frequency of said train of pulses.

4. A system for determining the pulse repetition frequency of a relatively short train of pulses comprising: an input circuit adapted to receive said train of pulses; delay means coupled to said input circuit for delaying said pulses a predetermined amount; gate means coupled to said input circuit for providing a trigger pulse responsive to an input pulse, said gate means being turned off responsive to reception of an input pulse; sweep voltage generating means coupled to said gate means for providing a sweep voltage responsive to a trigger pulse; voltage comparator means coupled to said sweep voltage generating means for providing a delayed pulse when said sweep voltage reaches a predetermined level; gate pulse generating means coupled to said voltage comparator means for generating gate pulses having a predetermined length responsiveto the delayed pulses, said gate pulse generating means being coupled to said gate means whereby the same is turned on for reception of another input pulse responsive to a said gate pulse; and coincidence means coupled to said gate pulse generating means and said input circuit for providing an output signal responsive to coincidence of an input pulse and a delayed pulse whereby the delay of said delay means indicates the repetition frequency of said train of pulses.

5. A system for determining the pulse repetition frequency of a relatively short train of pulses comprising: an input circuit adapted to receive said train of pulses; bistable multivibrator means coupled to said input circuit for providing a trigger pulse responsive to an input pulse, said multivibrator being incapable of retriggering responsive to further input pulses; delay means coupled to said multibigrator means for delaying said trigger pulses a predetermined amount; gate pulse generating means coupled to said delay means for generating gate pulses having a predetermined length responsive to the delayed pulses; said gate pulse generating means being coupled to said multivibrator means for retriggering the same for reception of another input pulse responsive to a said gate'pulse; and coincidence means coupled to said gate pulse generating means and to said input circuit for providing an output signal responsive to coincidence of an input pulse and a gate pulse whereby the delay of said delay means indicates the repetition frequency of said train of pulses.

6. A system for determining the pulse repetition frequency of a relatively short train of pulses comprising: an input circuit adapted to receive said train of pulses; bistable multivibrator means coupled to said input circuit for providing a trigger pulse responsive to an input pulse, said multivibrator being incapable of retriggering responsive to further input pulses; selectively adjustable delay means coupled to said multivibrator means for delaying the trigger pulse by an amount D T AT where D is the time delay, T is the predicted period of the input pulses and AT is a period at least one half the duration of one input pulse; selectively adjustable gate pulse generating means coupled to said delay means for generating gate pulses having a length ZAT responsive to the delayed pulses, said gate pulses being at least as long as said input pulses, said gate pulse generating means being coupled to said multivibrator means for regriggering the same for reception of another input pulse responsive to a said gate pulse; and coincidence means coupled to said gate pulse generating means and to said input circuit for providing an output signal responsive to coincidence of an input pulse and a gate pulse whereby the delay of said delay means indicates the repetition frequency of said train of pulses.

7. A system for determining the pulse repetition frequency of a relatively short train of pulses comprising: an input circuit adapted to receive said train of pulses; bistable multivibrator means coupled to said input circuit for providing a trigger pulse responsive to an input pulse, said multivibrator being incapable of retriggering responsive to further input pulses; sweep voltage generating means coupled to said multivibrator means for providing a sweep voltage responsive to a trigger pulse; voltage comparator means coupled to said sweep voltage generating means for providing a pulse having a predetermined delay when said sweep voltage reaches a predetermined level; gate pulse generating means coupled to said voltage comparator means for generating gate pulses having a predetermined length responsive to the delayed pulses; said gate pulse generating means being coupled to said multivibrator means for retriggering the same for reception of another input pulse responsive to a said gate pulse; and coincidence means coupled to said gate pulse generating means and to said input circuit for providing an output signal responsive to coincidence of an input pulse and a gate pulse whereby the delay of said delay means indicates the repetition frequency of said train of pulses.

8. A system for determining the pulse repetition frequency of a relatively short train of pulses comprising: an input circuit adapted to receive said train of pulses;

a bistable multivibrator having an input circuit coupled to said pulse train input circuit and having an output circuit for providing a step trigger pulse of predetermined polarity responsive to an input pulse, said multivibrator being incapable of retriggering responsive to further input pulses prior to reception of an opposite polarity retriggering pulse, a differentiating circuit coupling said multivibrator output circuit to the input circuit of a sweep voltage generator for providing a sweep voltage responsive to a differential trigger pulse; a voltage comparator having its input circuit coupled to the output circuit of said sweep voltage generator, said voltage comparator being arranged to compare a selectively adjustable reference voltage with said sweep voltage thereof to provide a delayed pulse having a predetermined delay D T-AT where D is the time delay, T is the predicted period of the input pulses and AT is a period at least one half the duration of one input pulse; a selectively adjustable gate pulse generator having its input circuit coupled to the output circuit of said voltage comparator and arranged to provide gate pulses having a length 2AT responsive to said delayed pulses, said gate pulses being at least as long as said input pulses; said gate pulse generator having one output circuit coupled to said input circuit of said multivibrator by another differentiating circuit and arranged to provide difierentiated gAte pulses thereto having a polarity opposite from said input pulses For retriggering said multivibrator for reception of another input pulse, and an AND circuit having one input circuit coupled to another output circuit of said gate pulse generator and having another input circuit coupled to said pulse train input circuit for providing an output signal responsive to coincidence of an input pulse and a gate pulse whereby the delay of said delay means indicates the repetition frequency of said train of pulses.

9. The combination of claim 1 further characterized in that said system is adjusted to a predetermined pulse period T where T is the predicted period of the input pulses, and further comprising a second system in accordance with claim I adjusted to a pulse period T/2; anti-coincidence means coupled to said second system and to receive said input pulses; and coincidence means coupled to said first system and to said anticoincidence means.

10. The combination of claim 2 further characterized in that said system is adjusted to a predetermined pulse period T, and further comprising a second system in accordance with claim 4 adjusted to a pulse period T12, the first and second systems having their input circuits coupled to receive the same input pulses; anticoincidence means having one input circuit coupled to receive said input pulses and having its other input circuit coupled to the output circuit of said second system for providing an output signal in the absence of coincidence of said input pulses and the output signal of said second system; and coincidence means having its input circuits respectively coupled to the output circuits of said anti-coincidence means and said first system for providing an output signal responsive to coincidence of the signals therein whereby said last-named output signal is provided only when said input pulses have a period T. 

1. A system for determining the pulse repetition frequency of a relatively short train of pulses comprising: an input circuit adapted to receive said train of pulses; selectively adjustable delay means coupled to said input circuit for delaying said pulses a predetermined amount; selectively adjustable gate pulse generating means coupled to said delay means for generating gate pulses having a predetermined length responsive to the delayed pulses; and coincidence means coupled to said gate pulse generating means and to said input circuit for providing an output signal responsive to coincidence of an input pulse and a gate pulse whereby the delay of said delay means indicates the repetition frequency of said train of pulses.
 2. A system for determining the pulse repetition frequency of a relatively short train of pulses comprising: an input circuit adapted to receive said train of pulses; delay means coupled to said input circuit for delaying the input pulses by an amount D T - Delta T where D is the time delay, T is the predicted period of the input pulses and Delta T is a period at least one half the duration of one input pulse; gate pulse generating means coupled to said delay means for generating gate pulses having a length 2 Delta T responsive to the delayed pulses, said gate pulses being at least as long as said pulses; and coincidence means coupled to said gate pulse generating means and to said input circuit for providing an output signal responsive to coincidence of an input pulse and a gate pulse whereby the delay of said delay means indicates the repetition frequency of said train of pulses.
 3. A system for determining the pulse repetition frequency of a relatively short train of pulses comprising: an input circuit adapted to receive said train of pulses; gate means coupled to said input circuit for providing a trigger pulse Responsive to an input pulse, said gate means being turned off responsive to reception of an input pulse; delay means coupled to said gate means for delaying the trigger pulses a predetermined amount; gate pulse generating means coupled to said delay means for generating gate pulses having a predetermined length responsive to the delayed pulses, said gate pulse generating means being coupled to said gate means whereby the same is turned on for reception of another input pulse responsive to a said gate pulse; and coincidence means coupled to said gate pulse generating means and said input circuit for providing an output signal responsive to coincidence of an input pulse and a gate pulse whereby the delay of said delay means indicates the repetition frequency of said train of pulses.
 4. A system for determining the pulse repetition frequency of a relatively short train of pulses comprising: an input circuit adapted to receive said train of pulses; delay means coupled to said input circuit for delaying said pulses a predetermined amount; gate means coupled to said input circuit for providing a trigger pulse responsive to an input pulse, said gate means being turned off responsive to reception of an input pulse; sweep voltage generating means coupled to said gate means for providing a sweep voltage responsive to a trigger pulse; voltage comparator means coupled to said sweep voltage generating means for providing a delayed pulse when said sweep voltage reaches a predetermined level; gate pulse generating means coupled to said voltage comparator means for generating gate pulses having a predetermined length responsive to the delayed pulses, said gate pulse generating means being coupled to said gate means whereby the same is turned on for reception of another input pulse responsive to a said gate pulse; and coincidence means coupled to said gate pulse generating means and said input circuit for providing an output signal responsive to coincidence of an input pulse and a delayed pulse whereby the delay of said delay means indicates the repetition frequency of said train of pulses.
 5. A system for determining the pulse repetition frequency of a relatively short train of pulses comprising: an input circuit adapted to receive said train of pulses; bistable multivibrator means coupled to said input circuit for providing a trigger pulse responsive to an input pulse, said multivibrator being incapable of retriggering responsive to further input pulses; delay means coupled to said multibigrator means for delaying said trigger pulses a predetermined amount; gate pulse generating means coupled to said delay means for generating gate pulses having a predetermined length responsive to the delayed pulses; said gate pulse generating means being coupled to said multivibrator means for retriggering the same for reception of another input pulse responsive to a said gate pulse; and coincidence means coupled to said gate pulse generating means and to said input circuit for providing an output signal responsive to coincidence of an input pulse and a gate pulse whereby the delay of said delay means indicates the repetition frequency of said train of pulses.
 6. A system for determining the pulse repetition frequency of a relatively short train of pulses comprising: an input circuit adapted to receive said train of pulses; bistable multivibrator means coupled to said input circuit for providing a trigger pulse responsive to an input pulse, said multivibrator being incapable of retriggering responsive to further input pulses; selectively adjustable delay means coupled to said multivibrator means for delaying the trigger pulse by an amount D T - Delta T where D is the time delay, T is the predicted period of the input pulses and Delta T is a period at least one half the duration of one input pulse; selectively adjustable gate pulse generating means coupled to said delay means for generating gate pulses having a length 2 Delta T responsive to the delayed pulsEs, said gate pulses being at least as long as said input pulses, said gate pulse generating means being coupled to said multivibrator means for regriggering the same for reception of another input pulse responsive to a said gate pulse; and coincidence means coupled to said gate pulse generating means and to said input circuit for providing an output signal responsive to coincidence of an input pulse and a gate pulse whereby the delay of said delay means indicates the repetition frequency of said train of pulses.
 7. A system for determining the pulse repetition frequency of a relatively short train of pulses comprising: an input circuit adapted to receive said train of pulses; bistable multivibrator means coupled to said input circuit for providing a trigger pulse responsive to an input pulse, said multivibrator being incapable of retriggering responsive to further input pulses; sweep voltage generating means coupled to said multivibrator means for providing a sweep voltage responsive to a trigger pulse; voltage comparator means coupled to said sweep voltage generating means for providing a pulse having a predetermined delay when said sweep voltage reaches a predetermined level; gate pulse generating means coupled to said voltage comparator means for generating gate pulses having a predetermined length responsive to the delayed pulses; said gate pulse generating means being coupled to said multivibrator means for retriggering the same for reception of another input pulse responsive to a said gate pulse; and coincidence means coupled to said gate pulse generating means and to said input circuit for providing an output signal responsive to coincidence of an input pulse and a gate pulse whereby the delay of said delay means indicates the repetition frequency of said train of pulses.
 8. A system for determining the pulse repetition frequency of a relatively short train of pulses comprising: an input circuit adapted to receive said train of pulses; a bistable multivibrator having an input circuit coupled to said pulse train input circuit and having an output circuit for providing a step trigger pulse of predetermined polarity responsive to an input pulse, said multivibrator being incapable of retriggering responsive to further input pulses prior to reception of an opposite polarity retriggering pulse, a differentiating circuit coupling said multivibrator output circuit to the input circuit of a sweep voltage generator for providing a sweep voltage responsive to a differential trigger pulse; a voltage comparator having its input circuit coupled to the output circuit of said sweep voltage generator, said voltage comparator being arranged to compare a selectively adjustable reference voltage with said sweep voltage thereof to provide a delayed pulse having a predetermined delay D T - Delta T where D is the time delay, T is the predicted period of the input pulses and Delta T is a period at least one half the duration of one input pulse; a selectively adjustable gate pulse generator having its input circuit coupled to the output circuit of said voltage comparator and arranged to provide gate pulses having a length 2 Delta T responsive to said delayed pulses, said gate pulses being at least as long as said input pulses; said gate pulse generator having one output circuit coupled to said input circuit of said multivibrator by another differentiating circuit and arranged to provide differentiated gAte pulses thereto having a polarity opposite from said input pulses For retriggering said multivibrator for reception of another input pulse, and an AND circuit having one input circuit coupled to another output circuit of said gate pulse generator and having another input circuit coupled to said pulse train input circuit for providing an output signal responsive to coincidence of an input pulse and a gate pulse whereby the delay of said delay means indicates the repetition frequency of said train of pulses.
 9. The combination of claim 1 fuRther characterized in that said system is adjusted to a predetermined pulse period T where T is the predicted period of the input pulses, and further comprising a second system in accordance with claim 1 adjusted to a pulse period T/2; anti-coincidence means coupled to said second system and to receive said input pulses; and coincidence means coupled to said first system and to said anti-coincidence means.
 10. The combination of claim 2 further characterized in that said system is adjusted to a predetermined pulse period T, and further comprising a second system in accordance with claim 4 adjusted to a pulse period T/2, the first and second systems having their input circuits coupled to receive the same input pulses; anti-coincidence means having one input circuit coupled to receive said input pulses and having its other input circuit coupled to the output circuit of said second system for providing an output signal in the absence of coincidence of said input pulses and the output signal of said second system; and coincidence means having its input circuits respectively coupled to the output circuits of said anti-coincidence means and said first system for providing an output signal responsive to coincidence of the signals therein whereby said last-named output signal is provided only when said input pulses have a period T. 